Currently, complex two-dimensional and three-dimensional scenes are described by scene graphs and small software programs that are downloaded to a terminal. A scene graph is a graphical representation of the contents of a scene. The scene graph determines, among other things, how three-dimensional objects are configured, where the video goes, where the audio goes, and how data is to be presented.
Programs are usually interpreted and executed at a terminal, but they require support for the program interpreter and program execution. This makes undesirable programs that are currently required to support meaningful scene descriptions.
Partial solutions exist for describing three-dimensional graphics content using scene graphs as described using Virtual Reality Modeling Language ("VRML"). The recent standards promulgated by the Motion Picture Experts Group, MPEG-4, extend this scene graph approach to describe natural and synthetic audio, video, and graphics scenes. These scene graphs do not provide any means by which the scene description could be automatically adapted to the terminal capabilities. This is because the known scene graph has no means of inquiring as to the terminal capabilities.
As an example, when rendering moving three-dimensional objects, the use of texture maps should be limited to terminals that support hardware-texture mapping. If a terminal does not support hardware-texture mapping, the render speed can easily drop below 2 frames per second, thus resulting in a lower-quality presentation than would otherwise be rendered.
If a content provider designs multimedia content for high-end terminals, it will be extremely difficult for a low-end terminal to present the content in an acceptable manner. Alternatively, if the content is designed for low-end terminals, using a high-end terminal will result in a poor-quality presentation.
To overcome these problems, VRML scene descriptions provide script capability. Scripts are small programs, executed at the terminal, that allow for both scene-graph animation and complicated interactions with the terminal. Scripts allow one to inquire as to a terminal's capabilities. Because scripts also enable arbitrary complex operations, however, the content provider is not able to design contents and scene graphs with a guaranteed level of performance in the terminal.
To avoid the overhead and unpredictable performance of the content presentation on a given terminal, the standards implemented in MPEG-4 do not implement scripts in its scene description language. Rather, what is required is a mechanism to enable scene-graph adaptation to the terminal without using scripts.
One such mechanism makes use of a backchannel. The backchannel enables interactivity with a server and allows the content presentation or scene graph to adapt to the terminal's capabilities. The use of a backchannel, however, requires additional protocols and infrastructure. For broadcast applications, however, the server is not able to adapt to the capabilities of each terminal but only to overall capabilities and events on the broadcast channel. Again, a mechanism is required that allows the content to adapt automatically to the terminal capabilities.